Predicting mass transfer in packed columns

Abstract

Countercurrent-flow columns are widely used in production processes in the chemical industry and their application in ecological engineering is of increasing importance. A theoretical model is presented here that allows mass transfer to be described in terms of packing geometry and physical properties which influence the gas-liquid or vapour-liquid systems in absorption, desorption and rectification columns. The relationships derived from the model can be applied to all countercurrent-flow columns, regardless of whether the packing has been dumped at random or arranged in a geometric pattern. Mass Transfer in the Liquid PhaseThe geometry and dimensions of modern packings are the main parameters that govern the flow of various phases and thus also the column efficiency. The liquid must flow in the form of a thin film and be distributed as uniformly as possible over the entire cross-section of the column in order to ensure large throughputs, effective mass transfer and moderate pressure drops. The surface of the packing should be wetted as much as possible and the countercurrent flow of gas should also be uniformly distributed over the column cross-section. Thus, the factors that govern the fluid dynamics and mass transfer of a column are the physical properties of the system, its capacity range and the shape and structure of the packing [I].The efficiency of a packing is influenced by the length of flow path I, which has to be traversed before the surface of the liquid in contact with the gas is renewed. Since the liquid is continually remixed at the points of contact with the packing, according to Higbie, the mass transfer in the liquid phase occurs by non-steady state diffusion, Eq. (1). DL is the diffusion coefficient of the transferring component and the time rL necessary for the renewal of interfacial area is determined by Eq. (2) Gravity and shear forces in the film are maintained at equilibrium with the frictional forces by the shear stress rv in the gas or vapour flow at the surface of the film, Eq. (4).In Eq. (4), ev is the gas density, iiv the average effective gas velocity and wL the drag coefficient for the gas-liquid or vapour-liquid countercurrent flow. (4)r,= -+wLeviiV 2 .Integration of Eq. (3) and substitution of the frictional force of the gas, acting at the surface of the liquid, by Eq. (4), lead theoretically to Eq. (5), valid for the liquid hold-up hL at and below the loading point [2-41. In Eq. (9, uL is the liquid load based on the column cross-section and a the total surface area of the packing.(2)Combining Eqs (I), (2) and (5) gives rise to Eq. (6) for the volumetric mass transfer coefficient &aph and Eq. (7) for the height of a transfer unit HTUL on the liquid-side, with CL being a constant, characteristic of the shape and structure of the packing 12, 5 , 61.If the packing is regarded as a large number of Chmnels through which the liquid of density QL and viscosity 4' L flows as a film with a local velocity iiL,s countercurrent to

LLNL ARRA Report on CCSI Uncertainty Quantification Capabilities with Applications on MEA Process Models

“…The two models were implemented within Aspen Plus; each represents a model of the absorber column with different assumptions on the chemical reactions and mass transfer correlations. The first model assumed chemical reactions at equilibrium and used the Billet and Schultes correlation (Billet and Schultes, 1993), while the second model assumed a mixture of equilibrium and rate-based reactions and used the Hanley structured correlation (Hanley and Chen, 2012).…”

Absorption of Sulphur Dioxide with Sodium Hydroxide Solution in Packed Columns

“…(3), whereby hydrogen sulfite reacts in turn with additional OH ± ions to sulfite as shown in Eq. (4). The reactionkinetic processes are also influenced by the dissociation of the water according to Eq.…”

“…They are dependent on the column loads u L , u V , the viscosities Z Z , Z V , the densities r L , r V , the diffusion coefficients for SO 2 D L , D V , the column hold-up h L , the hydraulic packing diameter d h , the specific surface area of the packing a, and the void fraction e as well as the interface a Ph . [2,3,4,15] …”

HETP evaluation of structured packing distillation column

“…A number of mass-transfer models, empirical or semitheoretical for packed columns, have been published in the literature [Bolles and Fair (1982), Billet and Mackowiak (1988), Billet and Schultes (1993), Bravo, Rocha and Fair (1985)]. …”